Shadows Wreak Havocs in Transition Disks

TL;DR

This paper shows that shadows on transition disks can induce eccentric structures and influence disk dynamics, potentially explaining observed features without requiring massive perturbers.

Contribution

It demonstrates through 3D simulations that asymmetric shadows can cause eccentric rings and affect disk evolution, a novel insight into disk morphology.

Findings

Shadows can truncate narrow rings sharply.

Shadows can split wide rings into eccentric rings.

Eccentric rings can drive gas accretion without viscosity.

Abstract

We demonstrate that shadows cast on a proto-planetary disk can drive it eccentric. Stellar irradiation dominates heating across much of these disks, so an uneven illumination can have interesting dynamical effects. Here, we focus on transition disks. We carry out 3D Athena++ simulations, using a constant thermal relaxation time to describe the disk's response to changing stellar illumination. We find that an asymmetric shadow, a feature commonly observed in real disks, perturbs the radial pressure gradient and distorts the fluid streamlines into a set of twisted ellipses. Interactions between these streamlines have a range of consequences. For a narrow ring, an asymmetric shadow can sharply truncate its inner edge, possibly explaining the steep density drop-offs observed in some disks and obviating the need for massive perturbers. For a wide ring, such a shadow can dismantle it into two (or possibly more) eccentric rings. These rings continuously exert torque on each other and drive gas accretion at a healthy rate, even in the absence of disk viscosity. Signatures of such twisted eccentric rings may have already been observed as, e.g., twisted velocity maps inside gas cavities. We advocate for more targeted observations, and for a better understanding on the origin of such shadows.